The present invention relates to a method of reacting a solution comprising a mixture of chemical compounds which are in chemical equilibrium with one another with at least one further chemical compound.
Chemically relevant substances in dissolved form are frequently present as a mixture of chemical compounds (components) which are in chemical equilibrium with one another. An industrially important example is aqueous solutions of formaldehyde. Formaldehyde is an important C1 building block in the chemical industry; world production is about 12 million metric tons per year. Formaldehyde is one of the most reactive organic components and is sold in aqueous solution in concentrations of from 20 to 55%, and is used for many chemical reactions. An aqueous formaldehyde solution comprises an equilibrium mixture of formaldehyde, methylene glycol and oligomers made up of at least two methylene glycol units, namely polyoxymethylene glycols. All these constituents are interconverted at a finite rate depending on, for example, pH and temperature. Thus, for example, the polyoxymethylene glycols are degraded to methylene glycol at elevated temperature.
Chemical reactions in industrial reactors generally do not proceed with a selectivity of 100%. However, the selectivity to the desired main product can be controlled by clever management of the reaction conditions. In the case of syntheses using formaldehyde, such a control opportunity is provided by the form in which the aqueous formaldehyde solution is employed. Thus, the selectivity to the main product can be optimized by, for example, appropriate setting of the concentrations of formaldehyde, methylene glycol and higher polyoxymethylene glycols.
Such a setting of the concentrations has hitherto only been possible within a narrow window by means of appropriate selection of the pH, the dilution and the temperature. The same applies to many other solutions which comprise a mixture of chemical compounds which are in chemical equilibrium with one another.
It is an object of the present invention to provide a method of reacting such solutions, which method makes it possible to set the concentration of particular components of the mixture in a targeted way and thus to control the selectivity of the chemical reactions which occur.
Such a method is proposed in DE-A-198 04 196, which has earlier priority but is not a prior publication, for the reaction of a formaldehyde solution comprising polyoxymethylene glycols and possibly monomeric formaldehyde and/or methylene glycol with aniline in the presence of acid catalysts.
We have found that the object of the present invention is achieved by a method of reacting a solution comprising a mixture of at least two chemical compounds which are in chemical equilibrium with one another with at least one further chemical compound, which comprises the following steps:
a) fractionation of the solution by means of a separation method to give at least two fractions in which different chemical compounds of the mixture are present in increased concentrations compared to the chemical equilibrium; and
b) reaction of one fraction with the further chemical compound or compounds before the chemical equilibrium is fully reestablished.
The use of a suitable separation method enables the concentrations of certain components of the mixture to be set in a targeted way within a very wide range. The concentrations of the components concerned are independent of parameters which can influence the course of the reaction in an undesirable manner, e.g. pH, dilution or temperature of the solution.
In order to ensure that the desired components in which the fraction has been enriched have not been reconverted into the other components which are in equilibrium with these components prior to the reaction, it is advantageous to make the time between the solution fractionation step and the reaction step shorter than the half-life time of the rate-determining step for establishment of the equilibrium between the components.
The separation method advantageously comprises at least the one step in which the solution is at least partly vaporized. The vaporization can advantageously be followed by an at least partial condensation.
The method of the present invention is particularly useful when the solution is a solution of an oligomer-forming substance in an appropriate solvent.
The method of the present invention can be particularly advantageously used when the solution is an aqueous formaldehyde solution comprising formaldehyde, methylene glycol and polyoxymethylene glycols made up of at least two methylene glycol units.
The fractions not employed in the reaction can, after appropriate further treatment, be once again treated by the method of the present invention. It is particularly advantageous for the equilibrium to be at least partially reestablished in at least one of the fractions not passed to the reaction by means of appropriate measures, e.g. a residence time apparatus, and for this fraction then to be returned to the separation step. In the simplest case, the residence time apparatus is a residence time vessel in which the fraction is left for a certain time.
To ensure uniform and favorable reaction conditions, it may be useful to remove solvent from the fraction introduced into the residence time apparatus. This can be done before or after this fraction passes through the residence time apparatus or while it is in the residence time apparatus.
For the fractionation step to give the different fractions, it is possible to use various separation methods. Particular advantages result from the fractionation being carried out using a thermal separation method in a film evaporator. Due to many control possibilities, various possible modes of operation and particular construction features, a film evaporator allows particularly precise setting of the concentrations of the components in the individual fractions. Furthermore, a film evaporator can be designed so that it can be scaled up or down without problems. Thus, the method of the invention can be used independently of scale in many applications.